1. Field of the Invention
The present invention relates to a resin composition for aqueous coating useful in cationic electro-coatings, in particular.
2. Description of the Related Art
Cationic electrocoatings each comprising, as the main components, a polyamine resin (e.g. amino-epoxy resin adduct) and a blocked polyisocyanate compound have been used in a large amount for their excellence in corrosion resistance, etc. These cationic electro-coatings, however, have various problems as listed below and solutions thereof are required strongly.
(1) The film of such a cationic electrocoating begins to cure at a temperature of 170.degree. C. or more, which is too high. PA1 (2) When the film of the cationic electro-coating is heated at high temperatures, the blocked polyisocyanate compound causes thermal decomposition and generates a resinous tar and soot, which allows the top coating film to give rise to yellowing, bleeding and insufficient curing, gives significantly reduced weather resistance and tends to cause whitening. PA1 (3) The cationic electrocoating generally uses an organotin compound as a catalyst for lowering the initial temperature of curing. This compound poisons, in some cases, catalysts for exhaust gas combustion used in passenger cars. PA1 (A) a resin having hydroxyl groups and cationic groups, and PA1 (B) a compound having, in the molecule, at least two glycidyl groups each in a glycidylamino group represented by the following formula (I) ##STR2## (R is a group selected from a hydrogen atom and a glycidyl group) directly bonding to carbon atoms of the aromatic ring. PA1 (1) Film curing is possible at a temperature of 160.degree. C. or less using no tin catalyst. Therefore, all the problems caused by the use of a tin catalyst can be eliminated. PA1 (2) No blocked polyisocyanate compound is required. Therefore, all the problems caused by the use of a blocked polyisocyanate compound can be eliminated. PA1 (3) Since there is no volume contraction due to thermal decomposition, a coating film having good surface smoothness can be formed. PA1 (4) Since neither urethane bond nor aromatic urea bond is formed in the crosslinking curing reaction, the corrosion resistance is not impaired. PA1 (5) The coating film is excellent in corrosion resistance, curability, etc. PA1 (6) Gives an electrocoating bath of excellent stability. PA1 (7) Film curing takes place quickly even under the baking conditions of low temperature and short time, whereby a coating film of excellent properties is formed. PA1 Component (A): a resin having hydroxyl group(s) and cationic groups in the molecule PA1 (1) A product obtained by reacting a polyepoxy resin with a cationizing agent. PA1 (2) A polycondensate between a polycarboxylic acid and a polyamine (U.S. Pat. No. 2,450,940). PA1 (3) A composition comprising (a) a product obtained by protonating a polyaddition product between a polyol and a mono- or polyamine with an acid and (b) a polyisocyanate compound. PA1 (4) A product obtained by protonating an acrylic or vinyl resin having hydroxyl group(s) and amino group(s), with an acid (U.S. Pat. Nos. 3,455,806 and 3,454,482). PA1 (5) A product obtained by protonating an adduct between a polycarboxylic acid resin and an alkyleneimine with an acid (U.S. Pat. No. 3,403,088). PA1 an epoxy resin (A-1) having, in the molecule, at least three epoxy group-containing functional groups each represented by the following formula (II) ##STR3## a primary or secondary amine compound (A-2) having at least one primary hydroxyl group in the molecule, and PA1 a phenol compound (A-3) having at least one phenolic hydroxyl group in the molecule. PA1 Component (A-1): an epoxy resin having, in the molecule, at least three epoxy group-containing functional groups represented by the above formula (II). PA1 Component (A-2): a primary or secondary amine compound having at least one primary hydroxyl group in the molecule. PA1 (1) Monoalkanolamines such as monoethanolamine, monopropanolamine, monobutanolamine and the like. PA1 (2) N-alkylalkanolamines and dialkanolamines such as N-methylethanolamine, N-ethylethanolamine, diethanolamine, di-n (or iso)-propanolamine, dibutanolamine and the like. PA1 (3) Adducts between monoalkanolamine and .alpha.,.beta.-unsaturated carbonyl compound, such as monoethanolamine-N,N-dimethylaminopropylacrylamide adduct, monoethanolamine-hydroxyethyl (meth)acrylate adduct, monoethanolamine-hydroxypropyl (meth)acrylate adduct, monoethanolamine-hydroxybutyl (meth)acrylate adduct and the like. PA1 (4) Monoalkanolaminoalkylamines such as hydroxyethylaminoethylamine and the like. PA1 (5) Condensates between (a) at least one compound selected from hydroxyethylamine, hydroxyethylhydrazine and hydroxybutrylhydrazine and (b) a ketone compound, for example, a dialkylketone such as dimethyl ketone, methyl ethyl ketone, methyl isobutyl ketone, dibutyl ketone, dipropyl ketone or the like. PA1 (6) Amine compounds having a primary hydroxyl group, a secondary amino group and an amido group in the molecule, represented by the following general formula (XVII) ##STR9## wherein n is an integer of 1-6 and R.sub.4 is a hydrocarbon chain of 4-36 carbon atoms which may have a hydroxyl group and/or a polymerizable unsaturated group. PA1 Component (A-3): a phenol compound having at least one phenoic hydroxyl group in the molecule. PA1 Component (B): a compound having, in the molecule, at least two glycidyl groups each in a glycidylamino group represented by the following formula (I) ##STR11## (R is a group selected from a hydrogen atom and a glycidyl group) directly bonding to carbon atoms of the aromatic ring.
Resins for self-crosslinking electrocoatings using no curing agent are also known and were proposed in, for example, GB-B-1327071, BG-B-1306101, BG-B-1306102, U.S. Pat. No. 4,001,101 and GB-B-1411249. None of these resins can satisfy both of the bath stability and film curability of electrocoating. Specifically, most common epoxy compounds of glycidyl ether type, for example, bisphenol A diglycidyl ether and novolac phenyl polyglycidyl ether, have excellent curability but inferior bath stability.
A composition using, as the curing agent, an epoxy resin having an alicyclic skeleton and/or a bridged alicyclic skeleton, which has been proposed by the present applicant in EP-A-356970, achieved most of the objects intended therein but is insufficient in film properties when baking was conducted at low temperatures for a short period of time.
The present inventors made an extensive study with a main aim of developing a resin composition for aqueous coating useful particularly in cationic electro-coatings, which uses neither blocked polyisocyanate compound nor organotin compound and which is free from the above-mentioned problems when an epoxy resin having an alicyclic skeleton and/or a bridged alicyclic skeleton is used as a curing agent. As a result, the present inventors found that a compound having, in the molecule, at least two glycidyl groups each in a glycidylamino group represented by general formula (I) (shown later) directly bonding to carbon atoms of the aromatic ring, is very useful as a curing agent for a resin having hydroxyl groups and cationic groups and can solve all of the above-mentioned problems. The finding has led to the completion of the present invention.
The curing agent identified by the present inventors does not substantially react, in an electro-coating bath at room temperature, with an acidic component (e.g. neutralizing agent), the hydroxyl group(s) of a base resin, etc. and resultantly gives excellent storage stability to the bath and further, as compared with the above-mentioned epoxy resin (curing agent) having an alicyclic skeleton and/or a bridged alicyclic skeleton, gives excellent low-temperature curability to the base resin. Moreover, the composition provided by the present invention requires neither blocked polyisocyanate compound nor organotin compound and is free from the above-mentioned problems caused by the use thereof.